Combustion characteristics of lignite char in a fluidized ...
Transcript of Combustion characteristics of lignite char in a fluidized ...
Combustion characteristics of lignite char in a fluidized bed
under O2/N2, O2/CO2 and O2/H2O atmosphere
Lin Li1, Lunbo Duan1,*, Shuai Tong1, Edward John Anthony2
1. Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of
Energy and Environment, Southeast University, Nanjing 210096, China
2. Centre for Combustion and CCS, School of Energy, Environment and Agrifood, Cranfield University,
Cranfield, Bedfordshire MK43 0AL, UK
*Corresponding author: Tel./fax: +86 (0) 25 83790147.
E-mail address: [email protected].
Abstract:Oxy-fuel combustion, O2/H2O combustion has many advantages over O2/CO2 combustion,
and has gradually gained more and more attention recently for carbon capture from coal-fired
power plant. The unique physicochemical properties (thermal capacity, diffusivity, reactivity)
of H2O will definitely influence the coal combustion process. In this work, the combustion
characteristics of lignite char were investigated in a fluidized bed combustor under O2/N2,
O2/CO2 and O2/H2O atmospheres with different oxygen concentration (15 %-27 %) and bed
temperature (837-937 oC). Results indicated that the average reaction rate (raverage) and the peak
reaction rate (rpeak) of lignite char in O2/H2O atmosphere was lower than that in O2/CO2
atmospheres at low oxygen concentrations. However, as the oxygen concentration increased,
the rpeak and raverage in O2/H2O atmosphere was significantly improved and exceeded that in
O2/CO2 atmosphere. The calculation of the activation energy based on the shrinking core model
showed that the order of activation energy under different atmospheres is: O2/CO2 (28.96
kJ/mol) > O2/H2O (26.11 kJ/mol) > O2/N2 (23.31 kJ/mol). Furthermore, with the increase of the
bed temperature, the active sites occupied by gasification agent were significantly increased,
the active sites occupied by oxygen decreased correspondingly.
Keywords: O2/H2O combustion; fluidized bed; lignite char; gasification; activation energy.
1. IntroductionOxy-fuel combustion technology (O2/CO2 combustion) is considered as a competitive
technology for carbon capture from coal-fired power plants and has attracted extensive research
in the past two decades [1]. Compared with conventional combustion, a mixture of pure O2 and
recycled flue gas is utilized as the oxidizer instead of air, resulting in more than 90% CO2
concentration in exhaust flue gas after condensation, which is conducive to CO2 separation [2,
3]. However, the low net efficiency and high cost is the biggest obstacle to the
commercialization of this technology [4].
In recent years, a new technical route of oxy-fuel combustion which used steam rather
than recycled flue gas to moderate the furnace temperature was proposed by Carlos [5], Seepana
and Jayanti [6]. In oxy-steam combustion, the flue gas recycle system can be removed, further
reducing the energy consumption and avoiding the air infiltration in the flue gas recycle lines.
In addition, it also have many advantages over O2/CO2 combustion as follows [5, 6]: 1) the
latent energy of water is much easier to recover owing to the enriched steam in the flue gas; 2)
the size of major and auxiliary equipment of the system is smaller than those of O2/CO2
combustion; 3) the formation of NOx and SOx can be decreased; 4) the power consumption of
pump and fan is relatively low because the transmission medium is water rather than flue gas.
Therefore, the oxy-steam combustion is identified as a promising next generation oxy-fuel
combustion. When this technology is introduced to the fluidized bed combustion, it will bring
more advantages such as realization of burning low-rank coal like lignite, uniform temperature
distribution and inherent low SO2/NOx emission.
Up to now, only few researches on the oxy-steam combustion have been published, which
were nearly all about pulverized coal (PC) furnaces. Jin et al. [7] investigated the process
characteristics of oxy-steam power plant by steady-state process model in Aspen Plus. It was
found that oxy-steam combustion exhibited better performance than O2/CO2 combustion on
both thermodynamic and economic aspects. Salvador et al. [8] proposed burner design for oxy-
steam combustion and conducted corresponding experiments in a 0.3 MWth oxy-fuel combustor.
It was concluded that oxy-steam combustion resulted in lower CO level, moderate NOx, typical
SOx, and 5~10% boosting CO2 concentration. Richards et al. [9] studied the residence time
requirements and the equilibrium CO levels of oxy-steam combustion through the combination
of experiments and simulations. The results showed that the residence time was 5~7 times
greater and the equilibrium CO levels were higher for the O2/CO2 combustion when compared with
O2/H2O combustion. Zou et al. [10-12] performed a series of experiments on the thermal
gravimetric analyzer and the drop tube furnace to study ignition and combustion characteristics
of PC in O2/N2 and O2/H2O atmospheres. The results showed that the ignition of PC in the
O2/H2O atmospheres occurred sooner than that in the O2/N2 atmospheres at identical oxygen
fractions. It can be attributed to the existence of the steam gasification reaction
(C+H2O→CO+H2) and the shift reaction (CO+H2O→CO2+H2) in H2O atmosphere. However,
fluidized bed (FB) combustion differs significantly from PC combustion in terms of
hydrodynamics, reaction kinetics and heat transfer.
Existing research for FB combustion were mainly on O2/CO2 atmosphere rather than
O2/H2O atmosphere. A 0.8 MWth oxy-fuel CFB boiler was carried out by Tan et al. [13],
achieving smooth transition between air-combustion and oxy-fuel combustion. Under stable
oxy-fuel combustion condition, CO2 concentrations can be reached to more than 90% (on a dry
basis), while the reduction of the emissions of NOx and SOx can be observed at the same time.
Similar conclusions were also obtained by Monica et al. [14] in a 30 MWth oxy-CFB Boiler.
Three kinds of fuel were burned in a 50 kWth oxy-fuel CFB combustor with warm flue gas
recycle has been reported by Duan et al. [15]. The results showed that an equivalent or higher
carbon burnout can be realized with a slightly higher O2 concentration (22.2%-23.4% for
different fuels) in oxy-fuel combustion. Scala and Chirone [16, 17] studied the combustion
characteristics of a single char particle in a FB reactor and found the Char-CO2 gasification was
the main reaction in O2/CO2 combustion. Additionally, a series of studies on combustion
characteristics of single coal particle also have been performed in a FB under O2/CO2
atmosphere [18-21].
As referred above, there has been no work published on oxy-steam fluidized bed
combustion of coal char, and the combustion mechanism has not been fully clarified for oxy-
steam combustion. Consequently, the aim of the present work is to evaluate the combustion
characteristics (such as the peak reaction rate, the burnout time and the average reaction rate)
and kinetic characteristics in FB under O2/H2O atmospheres, and quantitatively analyzing the
effect of gasification on combustion process.
2. Experimental
2.1 Materials
On account of lignite is a commonly used coal for fluidized bed boiler, a lignite
(Xiaolongtan lignite from China) was selected as the test coal in present work. The proximate
and ultimate analysis are shown in Table 1.
The fuel particle size of circulate fluidized bed boiler is generally 0-13 mm. However,
large coal particle combustion is significantly affected by diffusion, and too small particle is
easily carried out of the furnace by fluidized gas. Therefore, in order to evaluate the combustion
and kinetic characteristics of lignite char in fluidized bed, a relatively small particle size of test
coal (1.25~1.5 mm) was selected to minimize the influence of diffusion control region in this
paper. Before the test, the lignite need to be devolatilized in a fluidized bed reactor with N2
atmosphere at 900 °C for 7 min, then the char particle with size range of about 1~1.2 mm were
achieved.
Quartz sands (0.3-0.35 mm, 2560 kg/m3) were used as the bed material with unexpanded
bed height of 150 mm in the test, the corresponding minimum fluidization velocity (umf) was
0.042 m/s. The fluidized velocity (uf) was set at 0.126 m/s (uf /umf =3), corresponding the
bubbling condition.
Table 1 Properties of the coal samples.
Fuel
Proximate analysis, wt%
(as received)
Ultimate analysis, wt%
(dry and ash-free basis)
Moisture Volatile Fixed carbon Ash C H Oa N S
Lignite 16.17 35.53 39.18 9.12 67.11 4.23 25.07 1.45 2.14
a By difference
2.2 Experimental system
A lab-scale fluidized bed combustor was used in this study. The schematic diagram of the
experimental system is shown in Fig. 1. The system mainly contained four parts: the gas and
water supply line, the fluidized bed (FB) reactor, the temperature controlling system and the
data acquisition system of flue gas.
The flow of O2, N2 and CO2 from cylinders were controlled by three digital mass
flowmeters, respectively. A high-precision syringe pump was used to control the injection rate
of deionized water before heating in evaporator. Then the steam was mixed with O2 or N2 stream
as the fluidization gas fed into the combustor. The FB reactor was made of quartz glass with
inner diameter of 22 mm and length of 1200 mm, which included the preheating section with
the length of 500 mm. The preheating section and the reaction section were heated by 1 kW
and 5 kW electrical heaters, respectively. The furnace temperature was continuously controlled
within ±5 oC deviation by a PID controller, which was verified by the measurement of a moving
thermocouple. At the outlet of reactor, the flue gas was rapidly cooled and dried. Then the gas
products were analyzed by the online Fourier transform infrared multi-component gas analyzer
(with the resolution of 1ppm). In order to ensure that all kinds of gas products were in the
optimum measuring range of gas analyzer, a certain amount (1.2 L for O2/N2 atmosphere, 1.8
L for O2/CO2 atmosphere and 2.5 L for O2/H2O atmosphere) of N2 was used as a dilution gas
and fed into the gas analyzer at the same time.
N2 CO2 O2
Gas analyzer
Temperaturecontroller
Thermal insulation sectionSteamgenerator
Injectionpump
Mass
flow
con
troller
Gas mixer
Waterremoval
N2
Gasdistributor
Preheatingsection
Feedingport
Thermocouple
Deionizedwater
Fig. 1. The schematic diagram of experimental system.
2.3 Experimental procedure
After heating the FB combustor to the set temperature, the water and fluidizing gas were
introduced into reactor. 200 mg of char samples were injected into the reactor after the
fluidization condition reached a steady state. The tests were performed in O2/N2, O2/CO2,
O2/H2O, N2/CO2 and N2/H2O atmospheres with different oxygen concentrations (0%, 15%, 21%
and 27%) and bed temperatures (837 oC, 887 oC and 937 oC). The operating conditions are
summarized in Table 2. Each test was repeated at least three times to guarantee the good
repeatability. In fact, the results would be discarded and repeated if the carbon balance ratio
error of the test exceeded ±10%.
Before experiments, water balance tests were carried out by comparing the amount of
water injected in the pre-heater furnace (Vin) with the amount of condensate water in the outlet
of reactor (Vout). For all the tests, water balance ratios (Vout/Vin) were more than 98% within 2
hours.
Table 2 Operating conditions used in tests.
Fuel Tb (°C) Atmosphere O2 or N2 (%)
Lignite char
837, 887, 937O2/N2, O2/CO2,
O2/H2O21 (O2)
887O2/N2, O2/CO2,
O2/H2O15, 21, 27 (O2)
837, 887, 937 N2/CO2, N2/H2O 21 (N2)
887 N2/CO2, N2/H2O 15, 21, 27 (N2)
2.4 Data treatment
It should be noted that only a small amount of CH4 (less than 40 ppm) were measured in
O2/H2O atmospheres, so only the CO and CO2 were considered as the parameters in the carbon
balance calculation. Fig. 2 shows the typical concentration profiles of CO and CO2 for the
combustion test under different atmosphere. In O2/N2 and O2/CO2 atmospheres, char
combustion process can be described by three global reactions [17, 21]:
C + O2= CO2 ∆H= -394 kJ/mol (1)
C + CO2 = 2CO ∆H= +171 kJ/mol (2)
2CO + O2 = 2CO2 ∆H= -283 kJ/mol (3)
However, char combustion in O2/H2O atmospheres will occur additional reactions [11, 21]:
C + H2O = H2 + CO ∆H= +130 kJ/mol (4)
CO + H2O = H2 + CO2 ∆H= -40 kJ/mol (5)
2H2 + O2 = 2H2O ∆H= -242 kJ/mol (6)
The carbon conversion rate was calculated as following expression:
X= �WCO,i+WCO2,i�/WC,∞ (7)
where X is the carbon conversion rate; WCO,i and WCO2,i represent the weight of carbon in CO
and CO2 generated in the reaction time from 0 to i, respectively; WC,∞ represents the weight of
carbon generated during the whole reaction process. The amount of CO and CO2 can be
calculated as:
WCO,i= ∫ MC·CCO
i
0·Q/22.4·dt (8)
WCO2,i= ∫ MC·(CCO2
i
0-CCO2,0)·Q/22.4·dt (9)
where MC is the carbon molecular weight; t is the reaction time, CCO and CCO2 represent the gas
concentration of CO and CO2 measured by the gas analyzer; CCO2 is the CO2 concentration at
the inlet of reactor; Q is the flow rate at the outlet of reactor.
0 100 200 300 400 500 600
0
30000
60000
90000
0 100 200 300 400 500
0
30000
180000
2100000 50 100 150 200 250 300 350
0
20000
40000
60000
Time /s
CO CO2
Co
ncen
trat
ion
/pp
m
CO CO2
(c)
(b)
CO
(a)
CO2
Fig. 2 Typical CO and CO2 measured outlet profiles during char combustion tests: (a) O2/N2
combustion (T = 937 °C, 21% O2/79% N2); (b) O2/CO2 combustion (T = 937 °C, 21% O2/79%
CO2); (c) O2/H2O combustion (T = 937 °C, 21% O2/79% H2O).
2.5 Reliability analysis of the measurements
The reliability of the measurements was examined by carbon balance ratio which can be
calculated by
Carbon balance ratio=Wco,∞+ Wco2,∞
Wc,0×100% (10)
where the WC,0 is the carbon content of the sample particles. The carbon balance ratio of this
investigation are shown in Fig. 3. It indicated that the carbon balance ratio of all tests were in
range of 90% - 110%, which proved that the measurement was accurate and reliable.
15 20 25 30
80
90
100
110
120
Car
bon
bal
ance
rate
/%
O2
concentration / %
O2/N
2
O2/CO
2
O2/H
2O
Fig. 3. Carbon balance rate in different conditions.
3. Treatment method
3.1 Kinetic approach
The combustion reaction rate was described as
dX/dt= k·f(X) (11)
where f(X) is the reaction model; k is the reaction rate constant, which was calculated by
Arrhenius equation
k= Aexp(-E/RT) (12)
where A is pre-exponential factor; E is the activation energy; T is the reaction temperature; R is
the molar gas constant, 8.314 J/(mol·k). Rearranging the Eq. (11)
1
f(X)dX=kdt (13)
The gasification and combustion reaction kinetic models of coal is random and complex
among the numerous kinetic models. In this work, the shrinking core model was adopted to
calculate kinetic parameters of char combustion, which have been proven by many researchers
to be a suitable model for coal combustion and gasification research [22-24]. It was given by
G(X)= 1-(1-X)1/3 (14)
where G(X) is mechanism function model. Combining Eq. (13) with Eq. (14)
G(X)=1-(1-X)1
3=∫1
f(X)
X
0dX=∫ kdt
t
0=kt (15)
The k at different conditions could be calculated by the slope of Eq. (15). Then applying the
natural logarithm to Eq. (12),
ln k=ln A-E/RT (16)
The E and A were calculated by the slope (–E/R) and the intercept (lnA) of Eq. (16).
3.2 Calculation of carbon consumption rate
Generally, the differences of char combustion in O2/CO2, O2/H2O and O2/N2 atmospheres
are mainly brought by two aspects: (1) Gasification. The gasification reaction of char (such as
Eq. (1) and Eq. (4)) which cannot be neglected in the oxygen-fuel combustion (O2/CO2 and
O2/H2O); (2) Diffusivity of oxygen. The diffusivity of O2, CO2 and H2O in the binary gaseous
mixture are shown in Fig. 4. It is clear that the diffusivity of O2 in O2/H2O atmospheres is over
20% higher than that in O2/N2 atmospheres, and the diffusion rate of O2 in O2/CO2 atmosphere
is the lowest.
820 840 860 880 900 920 940
1.6
2.0
2.4
2.8
3.2
3.6
4.0
Dif
fusi
onco
effi
cien
t/m
2 /s
Temperature /°C
O2/N
2
O2/CO
2
O2/H
2O
N2/CO
2
N2/H
2O
Fig. 4. Diffusion rate in different atmospheres.
The simulated carbon consumption rate (vcal) could be calculated by accumulation of
carbon consumption rate of oxidation and gasification reactions. It needs to be pointed out that
the amount of quartz sands was much greater than that of test char, and the particle size of the
char was small, so good heat transfer between char and bed material ensured that their
temperatures are almost the same. The theory was also supported by Roy and Bhattacharya [21],
who carried out experiments and simulation studies on the single coal char combustion with
different particle sizes (≥1 mm) under oxy-fuel fluidized bed condition, and found that when
the size of char was 1 mm, the error caused by the assumption of equal particle and bed
temperatures was negligible. The vcal can be expressed as
vcal=voxi+ vgas (17)
where voxi and vgas are the reaction rate of oxidation and gasification, respectively. The voxi and
vgas were obtained by testing the char reaction rate in O2/N2, N2/CO2 and N2/H2O atmospheres
at the same partial pressure of reactant. Besides, the C-O2 oxidation is also affected by oxygen
diffusion in the boundary layer [25], so the reaction rate of oxidation in O2/CO2 and O2/H2O
atmospheres can be calculated by
voxi,O2/CO2= vO2/N2∙ DO2/CO2 DO2/N2⁄ (18)
voxi,O2/H2O= vO2/N2∙ DO2/H2O DO2/N2⁄ (19)
where voxi,O2/N2, voxi,O2/CO2, voxi,O2/H2O represent the reaction rate of oxidation in O2/N2, O2/CO2
and O2/H2O atmospheres, respectively; DO2/N2, DO2/CO2 and DO2/H2O represent the diffusivity of
oxygen in O2/N2, O2/CO2 and O2/H2O atmospheres, respectively. The reaction rate of
gasification in O2/CO2 and O2/H2O atmospheres can be given by
vgas,O2/CO2= vN2/CO2∙ DO2/CO2 DN2/CO2⁄ (20)
vgas,O2/H2O= vN2/H2O∙ DO2/CO2 DN2/H2O⁄ (21)
where vgas,N2/H2O, vgas,N2/CO2, vgas,O2/CO2 and vO2/H2O are represented the reaction rate of gasification
in N2/H2O, N2/CO2, O2/CO2 and O2/H2O atmospheres, respectively; DN2/CO2 and DN2/H2O
represent the diffusivity of CO2 or H2O in N2/CO2 and N2/H2O atmospheres, respectively.
The summation of carbon consumption rates of oxidation and gasification was also
compared with the carbon consumption rate from experimental conditions (vexp) of O2/CO2 and
O2/H2O atmospheres. The relationship between gasification and oxidization in char combustion
process can be expressed as follows:
(I) Mutual Promotion: vcal < vexp;
(II) Shared active site: vcal = vexp;
(III) Partial Shared activity site: voxi < vexp < vcal;
(IV) Mutual competition: vexp ≤ voxi.
4 Results and discussion
4.1 Carbon consumption rate of char
The reaction rate and carbon conversion rate of lignite char with various oxygen inlet
concentration under N2, CO2 and H2O atmospheres at different bed temperature are plotted in
Fig. 5 and Fig. 6. The general char combustion behavior in O2/N2 atmosphere was obtained:
After feeding the char sample into the reactor, it was ignited immediately and the reaction rate
was reached the peak rapidly. As the char was continuously consumed, the reaction rate
gradually decreased until the reaction was stopped. Interestingly, there were two peaks in
reaction rate curves in O2/CO2 and O2/H2O atmospheres while there was only one peak in O2/N2
atmosphere, and the peak of reaction rate in O2/H2O atmosphere were larger than that in O2/CO2
atmosphere at the same oxygen concentration. Furthermore, it could be found that the second
peak of reaction rate increased with the steam concentration in O2/H2O atmosphere, as shown
in Fig. 5c. This may be caused by the existence of gasification in CO2 and H2O environments.
With the advancement of reaction, the pore structure and specific surface area of char would be
greatly strengthened, and further promoting the gasification reaction. Due to the C-H2O reaction
is stronger than C-CO2 reaction at the same condition [26], the peak reaction rate in O2/H2O
atmosphere were larger than that in O2/CO2 atmosphere.
0 100 200 300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Time / s
Rea
ctio
nra
te/(
mg/
s)
837°C, 21O2
887°C, 15O2
887°C, 21O2
887°C, 27O2
937°C, 21O2
0 100 200 300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0837°C, 21O
2
887°C, 15O2
887°C, 21O2
887°C, 27O2
937°C, 21O2
Rea
ctio
nra
te/(
mg/
s)
Time / s0 100 200 300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0837°C, 21O
2
887°C, 15O2
887°C, 21O2
887°C, 27O2
937°C, 21O2
Rea
ctio
nra
te/(
mg
/s)
Time / s
(a)O2/N2 (b) O2/CO2 (c) O2/H2O
Fig. 5. Reaction rate in different conditions.
0 100 200 300 400 500 600 7000.00
0.25
0.50
0.75
1.00
837°C, 21O2
887°C, 15O2
887°C, 21O2
887°C, 27O2
937°C, 21O2
Car
bon
conv
ersi
onra
te
Time /s0 100 200 300 400 500 600
0.00
0.25
0.50
0.75
1.00
837°C, 21O2
887°C, 15O2
887°C, 21O2
887°C, 27O2
937°C, 21O2
Car
bon
conv
ersi
onra
te
Time /s
0 100 200 300 400 500 600 700 8000.00
0.25
0.50
0.75
1.00
837°C, 21O2
887°C, 15O2
887°C, 21O2
887°C, 27O2
937°C, 21O2
Car
bon
conv
ersi
onra
te
Time /s
(a)O2/N2 (b) O2/CO2 (c) O2/H2O
Fig. 6. Carbon conversion rate in different conditions.
4.2 Char combustion characteristics
Fig. 7 shows the peak reaction rate (rpeak), burnout time (tb) and average reaction rate
(raverage) with 21% of oxygen concentration under N2, CO2 and H2O atmospheres at different
bed temperature. It can be seen from Fig. 7 that with the increase of temperature, the tb
decreased, and rpeak and raverage increased. This result can be expected, because the gasification
and oxidation reactions are enhanced at high temperatures. In the same oxygen concentration
and bed temperature, the burnout time sequence was: O2/N2<O2/H2O<O2/CO2. The reason can
be attributed to the endothermic nature of the gasification, which will reduce the temperature
of char particles, thus reducing the reactivity of char. In addition, the diffusion of O2 in steam
(2.68 m2/s in 837°C) is 1.63 times larger than that in CO2 atmosphere (1.64 m2/s in 837 °C),
which will greatly promote the oxidation of char particle. So the tb in O2/H2O atmosphere was
shorter than that in O2/CO2 atmosphere.
The rpeak, tb and raverage with different oxygen concentration under N2, CO2 and H2O
atmospheres at 887 °C are plotted in Fig. 8. It can be seen that the effect of the oxygen
concentration on char combustion characteristics is similar to that of the bed temperature, while
the effect of changing oxygen concentration was more significant. The tb of char particle in
O2/H2O atmosphere was longer than that in O2/CO2 and O2/N2 atmospheres at low oxygen
concentration. However, as the oxygen concentration increases, the tb in O2/H2O atmosphere
was significantly improved and exceeded that in O2/CO2 atmosphere. There are two kinds of
effects of steam on the char combustion in O2/H2O atmospheres: 1) the steam-char gasification
reaction (Eq. (4)). 2) the steam shift reaction[11](Eq. (5)). At low oxygen concentration, higher
partial pressure of steam results in a stronger gasification reaction, which will significantly
reduce the temperature and reactivity of char particles. With the increase of oxygen
concentration, the gasification reaction is weakened and oxidation reaction is strengthened.
Furthermore, the steam shift reaction is an exothermic reaction, which is benefit to the char
combustion. Similar conclusions were also obtained by Zou et al. [11], who studied the ignition
behavior of pulverized coal in a drop tube furnace under O2/N2 and O2/H2O atmospheres.
840 860 880 900 920 940
0.6
0.8
1.0
840 860 880 900 920 940
150
200
250
300840 860 880 900 920 940
0.9
1.2
1.5
1.8
Temperature /oC
(c)r aver
age/
mg/s
t b/
sr p
eak
/m
g/s
(b)
O2/N
2
O2/CO
2
O2/H
2O
(a)
21%O2
Fig. 7 The rpeak, tb and raverage with 21% of oxygen concentration under N2, CO2 and H2O
atmospheres at different bed temperature.
14 16 18 20 22 24 26 280.4
0.6
0.8
1.0
1.214 16 18 20 22 24 26 28
150
200
250
300
35014 16 18 20 22 24 26 28
0.6
0.9
1.2
1.5
1.8
O2
concentration /oC
(c)r aver
age/
mg/s
t b/
sr p
eak
/m
g/s
(b)
887oC
O2/N
2
O2/CO
2
O2/H
2O
(a)
Fig. 8The rpeak, tb and raverage with different oxygen concentration under N2, CO2 and H2O
atmospheres at the bed temperature of 887 °C.
4.3 Analyses of combustion reaction kinetics
According to Eq. (15), the integral format of the reaction model G(X) should be
proportional to the reaction time at a constant temperature in theory. Combined with the
cumulative carbon conversion rate curve in Fig. 6, in order to minimize the influence of char
heating, char mixing and diffusion control region on reaction kinetics analysis in the
combustion process, the carbon conversion in the range of 0.1~0.85 was chosen to analyze
reaction kinetics in this work. Fig. 9 displayed the modeling and experiment results of char
combustion. It is indicated that the shrinking core model fitted the experiment results well in
different conditions.
According to Eq. (16), the activation energy (E) and pre-exponential factor (A) of lignite
char combustion were obtained by linear fitting and the results are shown in Fig. 10. The
calculation results of the kinetics parameters are given in Table 5. The results illustrated that
the order of activation energy under different atmospheres is: O2/CO2>O2/H2O>O2/N2. It is
widely accepted that the larger activation energy means the worse reactivity of reactants. So the
lignite char had the best reactivity in O2/N2 atmosphere and the worst reactivity in O2/CO2
atmosphere. It may be attributed that the char was oxidized and gasified simultaneously in
O2/CO2 and O2/H2O atmospheres, whereas only the oxidation reaction was occurred in O2/N2
atmosphere. There is a competitive mechanism between the oxidation and gasification reaction[27], and the heat absorption of gasification reduces the char particle temperature and inhibits
the activity of oxidation reaction.
50 100 150 200 250 300 350 4000.0
0.1
0.2
0.3
0.4
0.5
0.6
1-(
1-X
)1/3
Time /s
837 °C, 21O2
887 °C, 15O2
887 °C, 21O2
887 °C, 27O2
937 °C, 21O2
50 100 150 200 250 300 350 4000.0
0.1
0.2
0.3
0.4
0.5
0.6
837 °C, 21O2
887 °C, 15O2
887 °C, 21O2
887 °C, 27O2
937 °C, 21O2
1-(
1-X
)1/3
Time / s50 100 150 200 250 300 350 400
0.0
0.1
0.2
0.3
0.4
0.5
0.6
837 °C, 21O2
887 °C, 15O2
887 °C, 21O2
887 °C, 27O2
937 °C, 21O2
1-(1
-X)1
/3
Time /s
(a)O2/N2 (b) O2/CO2 (c) O2/H2O
Fig. 9 Fitting results of char combustion using shrinking core model in different conditions.
0.82 0.84 0.86 0.88 0.90 0.92-6.8
-6.4
-6.0
-5.6
-5.2O
2/N
2
O2/CO
2
O2/H
2O
lnk
1000/T / K-1
Fig. 10 Fitting results between reaction rate and temperature.
Table 3 Kinetics parameters of lignite char combustion.
Atmosphere E (kJ/mol) A (s-1) R2
21%O2/79%N2 23.31 0.0332 0.992
21%O2/79%CO2 28.96 0.0366 0.999
21%O2/79%H2O 26.11 0.0313 0.985
4.4 Quantitative analysis of gasification and oxidation reaction
4.4.1 Effect of CO2 and H2O in feed gas
The variation of average carbon reaction rate versus bed temperature and oxygen
concentrations in O2/CO2 and O2/H2O atmospheres are plotted in Fig. 11 and Fig. 12. It is
obviously that the calculated total carbon conversion rates were significantly larger than that of
the experimental results, and the experimental carbon conversion rates were even lower than
the calculated carbon conversion rates of oxidation. It can be concluded that competition
reaction exists between the gasification and oxidation reaction in the char combustion process
under O2/CO2 and O2/H2O atmospheres. Fig. 11 also shown that the difference between
calculated results and experimental results increase with the bed temperature. This is because
the reaction rate of gasification and oxidation increased significantly with the increase of bed
temperature, which would cause more intense competition between them. Otherwise, the partial
pressure of CO2 and H2O decreased with the increase of oxygen concentration, as a
consequence the gasification reaction rate slightly decreased at high oxygen concentration, and
the effect of gasification on the combustion progress reduced. The detailed quantitative analysis
of the proportions of gasification and oxidation reaction in char combustion progress will be
shown in 4.4.2.
820 840 860 880 900 920 9400.0
0.2
0.4
0.6
0.8
1.0
1.2 21%O2/79%CO
2
Rea
ctio
nra
te/m
g/s
Temperature /°C
Calculated: GasificationCalculated: OxidationCalculated: totalExperimental
12 16 20 24 28
0.2
0.4
0.6
0.8
1.0
1.2Calculated: GasificationCalculated: OxidationCalculated: totalExperimental
887°C, O2/CO
2
Rea
ctio
nra
te/m
g/s
Oxygen concentration /%
(a) 21%O2/79%CO2 (b) 887 °C
Fig. 11 Experimental and calculated average carbon conversion rate in different conditions
under O2/CO2 atmosphere.
820 840 860 880 900 920 9400.0
0.4
0.8
1.2
1.6
2.0
Rea
ctio
nra
te/m
g/s
Temperature /°C
21%O2/79%H
2OCalculated: Gasification
Calculated: OxidationCalculated: totalExperimental
12 16 20 24 280.0
0.4
0.8
1.2
1.6
2.0887°C, O
2/H
2OCalculated: Gasification
Calculated: OxidationCalculated: totalExperimental
Rea
ctio
nra
te/m
g/s
Oxygen concentration /%
(a) 21%O2/79%H2O (b) 887 °C
Fig. 12 Experimental and calculated average carbon conversion rate in different conditions
under O2/H2O atmosphere.
4.4.2 Quantitative analysis
The average combustion rate of char in O2/CO2 and O2/H2O atmospheres can be expressed
as:
vO2/CO2= a·vN2/CO2 + b·vO2/N2 (22)
vO2/H2O= a·vN2/H2O + b·vO2/N2 (23)
a+b=1 (24)
where the a and b are represented the active sites occupied by gasification agent (CO2 or H2O)
and oxidant (O2) on the char surface in the combustion progress, respectively. The variation of
a and b versus bed temperature and oxygen concentrations in O2/CO2 and O2/H2O atmospheres
are showed in Table 4, and the corresponding reaction rate are displayed in Table 5. It can be
found that the reaction rate of gasification and oxidation both increased with the increase of
bed temperature, this is attributed to the higher reactivity of char at high temperature. At the
same oxygen concentration, the active sites occupied by gasification agent significantly
increased with bed temperature, and the active sites occupied by oxygen decreased
correspondingly. The phenomenon indicated that when the char was burned in O2/CO2 or
O2/H2O atmosphere, the inhibitory effect of the gasification reaction on the oxidation reaction
was strengthened as the increase of temperature. Moreover, it is noticed clearly that the
proportion of gasification reaction in O2/H2O atmospheres were higher than that in O2/CO2
atmospheres. This is due to the activity of C-H2O reaction is better than that of C-CO2 reaction.
Interestingly, the voxi and (voxi+vgas) in O2/H2O atmosphere were all lower than that in O2/CO2
atmosphere at low oxygen concentration. However, with the increase of oxygen concentration,
the voxi and (voxi+vgas) in O2/H2O atmosphere were remarkably strengthened and exceeded that
in O2/CO2 atmosphere. The results indicated that the char combustion rate in O2/H2O
atmosphere shows greater advantages at high oxygen concentrations than that in O2/CO2
atmosphere.
Table 4. Values of the a and b
837°C 887°C 887°C 887°C 937°C
O2/CO2 21%O2 15%O2 21%O2 27%O2 21%O2
a 0.193 0.065 0.211 0.300 0.293
b 0.807 0.935 0.789 0.700 0.707
O2/H2O 21%O2 15%O2 21%O2 27%O2 21%O2
a 0.474 0.617 0.554 0.493 0.575
b 0.526 0.383 0.446 0.507 0.425
Table 5. Gasification and oxidation reaction rates during char combustion
837°C 887°C 887°C 887°C 937°C
O2/CO2 21%O2 15%O2 21%O2 27%O2 21%O2
Vgas 0.019 0.014 0.045 0.065 0.089
voxi 0.511 0.443 0.534 0.606 0.559
O2/H2O 21%O2 15%O2 21%O2 27%O2 21%O2
Vgas 0.077 0.147 0.120 0.101 0.175
voxi 0.580 0.297 0.494 0.718 0.549
5 Conclusion
The goals of this study are to obtain the combustion characteristics of lignite char in a
fluidized bed under O2/H2O, O2/CO2 and O2/N2 atmospheres. The main conclusions can be
drawn as follows:
(1) With the increase of temperature and O2 concentration, the similar combustion
characteristics of lignite char were exhibited in O2/H2O, O2/CO2 and O2/N2 atmospheres (the tb
decreased, while rpeak and raverage increased).
(2) At low oxygen concentration, the combustion characteristics of lignite char in O2/H2O
atmosphere was worse than that in O2/CO2 and O2/N2 atmospheres; however, as the oxygen
concentration increases, the combustion characteristics in O2/H2O atmosphere was significantly
improved and exceeds that in O2/CO2 atmosphere.
(3) The calculation results showed that the relationship among activation energy under different
atmospheres is: O2/CO2 (28.96 kJ/mol) > O2/H2O (26.11 kJ/mol) > O2/N2 (23.31 kJ/mol).
(4) The active sites occupied by gasification agent significantly increased as the bed
temperature increases, and the competitive effect of gasification on oxidation is enhanced.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(NO. 51776039).
References
[1] Buhre B J P, Elliott L K, Sheng C D, et al. Oxy-fuel combustion technology for coal-fired
power generation[J]. Progress in Energy & Combustion Science, 2005, 31(4): 283-307.
[2] Wall T F. Combustion processes for carbon capture [J]. Proceedings of the Combustion
Institute, 2007, 31(1): 31-47.
[3] Toftegaard M B, Brix J, Jensen P A, et al. Oxy-fuel combustion of solid fuels [J]. Progress
in Energy & Combustion Science, 2010, 36(5): 581-625.
[4] Lasek J A, Janusz M, Zuwała J, et al. Oxy-fuel combustion of selected solid fuels under
atmospheric and elevated pressures [J]. Energy, 2013, 62(6): 105-112.
[5] Carlos S. Modeling design and pilot-scale experiments of CANMET’S advanced oxy-
fuel/steam burner. USA: International Oxy-Combustion Research Network; 2007.
[6] Seepana S, Jayanti S. Steam-moderated oxy-fuel combustion [J]. Energy Conversion &
Management, 2010, 51(10): 1981-1988.
[7] Jin B, Zhao H, Zou C, et al. Comprehensive investigation of process characteristics for
oxy-steam combustion power plants[J]. Energy Conversion & Management, 2015, 99: 92-
101.
[8] Salvador C, Mitrovic M, Kourash K. Novel oxy-steam burner for zero-emission power
plants. http://www.ieaghg.org/docs/oxyfuel/OCC1/Session%206_C/2_NOVEL%20OXY-
TEAM%20BURNER%20FOR%20ZERO-EMISSION%20POWER%20PLANTS.pdf >.
[9] Richards G A, Casleton K H, Chorpening B T. CO2 and H2O diluted oxy-fuel combustion
for zero-emission power [J]. Institute of Mechanical Engineers Part A Power & Energy,
2005, 219(2): 121-126.
[10] Lei C, Zou C, Yang L, et al. Numerical and experimental studies on the ignition of
pulverized coal in O2/H2O atmospheres[J]. Fuel, 2015, 139: 198-205.
[11] Zou C, Cai L, Wu D, et al. Ignition behaviors of pulverized coal particles in O2/N2, and
O2/H2O mixtures in a drop tube furnace using flame monitoring techniques [J].
Proceedings of the Combustion Institute, 2015, 35(3): 3629-3636.
[12] Zou C, Zhang L, Cao S, et al. A study of combustion characteristics of pulverized coal in
O2/H2O atmosphere [J]. Fuel, 2014, 115: 312-320.
[13] Tan Y, Jia L, Wu Y, et al. Experiences and results on a 0.8 MWth oxy-fuel operation pilot-
scale circulating fluidized bed [J]. Applied Energy, 2012, 92(4): 343-347.
[14] Monica Lupion, Iñaki Alvarez, Pedro Otero, et al. 30 MWth CIUDEN Oxy-CFB Boiler-
First Experiences [J]. Energy Procedia, 2013, 37: 6179-6188.
[15] Duan L, Sun H, Zhao C, et al. Coal combustion characteristics on an oxy-fuel circulating
fluidized bed combustor with warm flue gas recycle [J]. Fuel, 2014, 127(2): 47-51.
[16] Scala F, Chirone R. Combustion of Single Coal Char Particles under Fluidized Bed
Oxyfiring Conditions [J]. Industrial & Engineering Chemistry Research, 2009,
49(21):11029-11036.
[17] Scala F, Chirone R. Fluidized bed combustion of single coal char particles at high CO2,
concentration [J]. Chemical Engineering Journal, 2010, 165(3): 902-906.
[18] Bu C, Bo L, Chen X, et al. Devolatilization of a single fuel particle in a fluidized bed under
oxy-combustion conditions. Part A: Experimental results [J]. Combustion & Flame, 2015,
162(3): 797-808.
[19] Bu C, Bo L, Chen X, et al. Devolatilization of a single fuel particle in a fluidized bed under
oxy-combustion conditions. Part B: Modeling and comparison with measurements [J].
Combustion & Flame, 2014, 162(3): 809-818.
[20] Bu C, Pallarès D, Chen X, et al. Oxy-fuel combustion of a single fuel particle in a fluidized
bed: char combustion characteristics, an experimental study [J]. Chemical Engineering
Journal, 2016, 287(3): 649-656.
[21] Roy B, Bhattacharya S. Combustion of single char particles from Victorian brown coal
under oxy-fuel fluidized bed conditions [J]. Fuel, 2016, 165: 477-483.
[22] Sadhukhan A K, Gupta P, Saha R K. Analysis of the dynamics of coal char combustion
with ignition and extinction phenomena: Shrinking core model [J]. International Journal
of Chemical Kinetics, 2008, 40(9): 569-582.
[23] Zhang L, Huang J, Fang Y, et al. Gasification Reactivity and Kinetics of Typical Chinese
Anthracite Chars with Steam and CO2 [J]. Energy & Fuels, 2006, 20(3): 1201-1210.
[24] Chen C, Wang J, Liu W, et al. Effect of pyrolysis conditions on the char gasification with
mixtures of CO2, and H2O [J]. Proceedings of the Combustion Institute, 2013, 34(2): 2453-
2460.
[25] Wang W, Bu C, Gómez–Barea A, et al. O2/CO2 and O2/N2 combustion of bituminous char
particles in a bubbling fluidized bed under simulated combustor conditions [J]. Chemical
Engineering Journal, 2018, 336: 74-81.
[26] Roberts D G, Harris D J. Char gasification in mixtures of CO2 and H2O: Competition and
inhibition [J]. Fuel, 2007, 86(17): 2672-2678.
[27] Senneca O, Cortese L. Kinetics of coal oxy-combustion by means of different experimental
techniques [J]. Fuel, 2012, 102: 751-759.